1. Field of Invention
The present invention relates to an information hiding method and, in particular, to an information hiding method with reduced fuzziness.
2. Related Art
Owing to the rapid development in computer network technology, the whole world has entered the digital era. This has made information communications and knowledge sharing much easier. However, the issue of security over the network gradually threatens the efforts in protecting network intellectual properties. The applications of computer networks in national defense, such as the information systems in command, management, communications and intelligence or modern weapon equipment, are more and more popular. Therefore, how classified information can acquire safety authenticates on the network has become an urgent problem to be solved. Practically speaking, the so-called electronic signature technology refers to the techniques that prevent unintended users from eavesdropping, copying and modifying others' information so that the legal user can extract authenticated information from a meaningful image. This is the so-called steganography.
By steganography refers to embedding a portion of meaningfull texts, image or video signal irrelevant to the image being transmitted during the image transmission. It is made to be hard for an unauthorized third party to see whether there is hidden information or not in the transmission process. This can prevent hackers from damaging the transmitted information. A common method seen in the steganography is to select a host image, to hide therein some information and thus to generate another watermarked image. This image looks just like the host image by naked eyes, and it is hard to find out directly that other information is contained therein. Other users on the network cannot determine whether this signed image contains other information or not. Thus, from all the information on the network unauthorized users are unable to distinguish electronically signed images from unsigned ones. However, a legal user can readily extract the hidden electronic signature from the transmitted information.
Normally, the steganography must have the following features:                1. Undetectable: The electronic signature is hidden behind the image information so that it would not be found using usual image processing methods;        2. Invisible: An image attached with the electronic signature looks the same as the host image by naked eyes;        3. Undeletable: The electrical signature added to the image cannot be easily deleted using simple image processing methods;        4. Resistant to image manipulation: The electrical signature is not susceptible to damages caused by normal image processing or on purpose.        
The steganography has the space and frequency domains. In the research field of the frequency domain, using frequency expansions on the electronic signature is fine but has the following three disadvantages:                1. The computation is too tedious. Conversion to the frequency domain requires complicated calculation. The receiver end also needs the corresponding converter. This is inconvenient to applications that demand real time processing.        2. It is vulnerable to attacks. Most hiding methods in the frequency domain hide the information in peripheral sections to avoid damages caused by fuzziness compression. Therefore, it is easy for invaders to attack the hidden information.        3. It can hide relatively little information. The information can only be hidden within a specific frequency band in the frequency domain, so it can hide relatively little information.        
In the space domain, a commonly employed information hiding method is the vector quantization. It uses a code book commonly owned by both the sender and the receiver to encode the electronic signature. Over the sender end, the sender cuts the electronic signature to be hidden into the same size as the blocks in the code book, finds a similar block from the code book and hides in the image its index in place of the information in this block, and finally sends it to the network. Over the receiver end, when the receiver receives the image with hidden information he decodes the index, looks it up in the code book and restores the information. The defect of this method is that there is larger information fuzziness and that the image with hidden information cannot withstand the damages caused by fuzziness.
Another steganography uses fixed areas in image pixels to hide information, e.g., the fixed range equalization method. The gray scale of the image to hide information in this method is divided into 16 sections. The hidden information, in unit of bytes, replaces the original value at a point depending upon the space the input value belongs to. For example, if the information has a value of 17, then one point within the range from 16˜31 is thus replaced by 17. There is yet another method called LSB, which is also often used for hiding information. LSB places the electronic signature in the lower (less important) bytes because the change in lower bytes has less impact on the whole pixel values and thus has less obvious damages to the image. Nevertheless, the above two methods cannot withstand the fuzziness damages after the image is hidden with information. Other references provide some ingenious methods for hiding information, but none of them can provide an effect solution to human damages.